This invention relates generally to multispectral imaging systems, and more particularly to imaging systems capable of producing a composite visible/thermal-infrared image of an object or scene.
The use of passive infrared (IR) thermal imaging (thermography) for applications such as non-destructive testing, medical diagnosis, surveillance and military target identification, and guidance and tracking applications has been hampered to some extent by the difficulty operators have in positively recognizing and locating objects in a thermal image. Even familiar objects are often difficult to recognize from images produced by the object's passively emitted thermal radiation in the infrared spectral region. Under the pressures of simulated or actual combat, even experienced pilots have been known to fire at a "hot" feature revealed by their FLIR (Forward Looking Infrared) display system without being able to identify the target. Similar problems occur whenever infrared images are interpreted without familiar visual cues. Even the highest resolution thermal imaging systems can do little to improve this situation because of fundamental differences in the behavior of matter in the visible and infrared spectral regions. For example, glass and water are transparent to the human eye, but are virtually opaque in the thermal infrared region. Paints and semiconductors are visually opaque, but are partially transparent in the infrared. A further complication is that all matter glows to some extent at infrared wavelengths, and an infrared image is, in effect, a radiant temperature map of a scene.
Multispectral imaging systems which produce a composite visual/infrared image of a scene are known. Such systems have the advantage of combining on a single display visual and thermal information, and are useful in enabling thermal features in a complex scene to be positively located and identified. Known systems, however, suffer from several problems, a principal one being obtaining exact spatial registration of the visual and infrared images. In systems which employ separate optical, detection and image processing and display systems for the visible and thermal-infrared spectral bands, some degree of parallax is unavoidable. Although overlaid multispectral images may be formed using digital image processing techniques, it is very difficult to achieve exact spacial registration in current systems. Also, digital processing of high resolution images is usually not performed in real time because of the extensive computations required. Even systems which have a common optical train have difficulty in achieving spatial registration of the images due to difficulty in focusing the widely separated visual and infrared wavelength bands.
It is desirable to provide composite visual/thermal-infrared imaging systems which avoid these and other problems of known systems, and it is to this end that the present invention is directed.